ABSTRACT Organophosphorus (OP) insecticides are ubiquitously used worldwide, with chlorpyrifos (CPF) leading the markets for decades. Acute OP poisoning, the focus of the parent R01 grant, is well-described and is primarily, though not exclusively, due to the irreversible inhibition of acetylcholinesterase (AChE), the enzyme that hydrolyzes the neurotransmitter acetylcholine (ACh). Equally concerning, however, are the detrimental health effects associated with exposures to OP insecticide levels that do not cause marked AChE inhibition and do not trigger acute intoxication (referred to as subacute OP exposures). Meta analyses of epidemiological studies have identified subacute OP exposures as a risk factor for Alzheimer?s disease (AD); however, they could not determine the sex dependence of the risk. In addition, the population-based studies were not suitable to establish a cause-consequence relationship between the OP exposures and the development of AD and to determine the identity of the OP insecticides associated with risk for AD. Earlier preclinical studies reported that CPF and its oxon metabolite aggravate AD-related neuropathology in male transgenic mice carrying gene mutations associated with familial AD (FAD), which accounts for <10% of all cases of AD. Sporadic AD (SAD), which accounts for >90% of all AD cases, has a late onset and a complex, heterogeneous genetic component. Yet, to date, no preclinical study has demonstrated whether subacute CPF exposures can precipitate development of SAD. In addition, although in clinical and preclinical studies, males appear to be more sensitive than females to the neurotoxic effects of CPF, it remains unknown whether the ability of subacute CPF exposures to trigger AD-related neuropathology is sex dependent. The present study is, thus, designed to use male and female guinea pigs ? a proposed model of SAD ? to test the hypothesis that subacute CPF exposures during adolescence, a vulnerable period of brain growth, lead to the gradual development of memory deficits that are accompanied by AD-related neuropathology and overexpression of AD-related genes in adulthood. A translational, multidisciplinary approach that involves behavioral assays, in vivo imaging, and biochemical, molecular biological, and immunohistochemical assays, in addition to a randomized, blind design that maximizes scientific rigor and minimizes bias will be used to address the test hypothesis. Successful completion of this study will determine whether subacute CPF exposures at a well-defined, vulnerable period of brain growth induce AD-related neuropathology in a preclinical model of SAD, and, if so, whether the effect is sex dependent. Results of this study will be far- reaching as they will lay the groundwork for identification of structural and/or metabolic biomarkers critically needed to diagnosticate AD in its early (subclinical) stages and to follow its progression. In addition, the establishment of a preclinical model of CPF-precipitated SAD pathology will be pivotal for future work to identify new molecular targets for development of pharmacotherapies to treat AD.